Method of making magnetic cores



March 1, 1955 w, c HURT, JR

METHOD OF MAKING MAGNETIC CORES Filed April 7, 1952 Inventor. William C.Hurb,dr-., y 1

United States Patent METHOD OF MAKING MAGNETIC CORES William C. Hurt, Jr., Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Application April 7, 1952, SeriaI No. 281,007 3 Claims. (Cl. 29-15558) This invention relates to magnetic cores for stationary electrical induction apparatus and more particularly to magnetic cores of the radially-nested bent strip type formed of oriented magnetic material, and a method of assembling preformed windings on such a core.

Magnetic cores having straight legs and curved yokes formed of strip magnetic material and having a plurality of superposed layers of flatwise bent or curved strip magnetic material in which the advantageous magnetic direction is substantially parallel to the lengthwise edges of the strips have come into widespread use in the electrical industry. These cores have the advantage that the magnetic flux substantially always travels with the grain of the magnetic material with resultant low core losses.

In order to place preformed electrical windings on the legs of such a core it is necessary that each of the magnetic strip turns be cut in at least one place in order to permit opening of the core for placement of the electrical windings. This invention is directed particularly to magnetic cores of bent strip magnetic material in which only a single joint is employed in each of a plurality of layers of the magnetic material. By using only one joint in a layer, the number of laminar members per layer is reduced to a minimum, thereby facilitating assembly of the core. Furthermore, by reducing the number of joints to only one per layer, core losses and exciting current due to joints in the magnetic circuit are reduced to a low value.

While not restricted thereto, my invention has particular utility in connection with magnetic cores formed from a plurality of precut flat strips of magnetic material which are cut to predetermined lengths equal to the perimeter of the core at the positions to be occupied by the respective strips and subsequently bent around upon themselves to form a closed magnetic circuit, with each layer of the core consisting of a single strip which is bent back upon itself to form a closed loop in which the two opposite ends of the strip are jointed to each other.

In one arrangement which has been used in magnetic cores of bent strip magnetic material, all of the strips are jointed in substantially the same region of one of the core legs. When the core is opened up at this joint region, it is very simple to place an electrical winding on the core leg in which all of the joints are positioned. However, if it is desired to slip an electrical winding in place simultaneously over all the strip members of the leg which is unjointed or uncut, considerable difiiculty is sometimes experienced due to the difliculty encountered in bending all the strips so that they extend substantially straight out from the leg on which the preformed electrical winding is being placed, and then positioning a preformed electrical winding on the strip portions extending from the core leg. This necessitates attempting to flatten out the previously curved corners so that the cross section of the opened-up strips is substantially the same at the corners as in any portion of the unjointed leg itself to permit positioning of the electrical winding on the unjointed leg. This procedure obviously presents difiiculties and is not a very practical solution of the problem.

Accordingly, it is an object of this invention to provide a new and improved joint arrangement for magnetic cores having a plurality of layers of strip magnetic material in which each layer is jointed in only one place.

It is a further object of this invention to provide a new and improved structure for magnetic cores comprising a plurality of superposed layers of bent strip magnetic ma- 'ice terial in accordance with which each layer has only one It is a still further object of this invention to provide a joint arrangement which has greater mechanical strength than a joint arrangement in which all joints are.

located in a relatively short length of the core.

It is another object of this invention to provide a new method of assembling preformed electrical windings on magnetic cores of the bent strip type having only one joint per layer.

In accordance with these objectives, this invention provides a joint arrangement and method of assembly for. use with a magnetic core comprising a plurality of layers of bent strip magnetic material in which only one joint is provided in any given layer with the joints for the plurality of layers being so distributed that a substantial proportion of the joints lies in each member of a pair of oppositely disposed leg members. In accordance with my method of assembling preformed electrical windings on such a core, the bent strip magnetic core is first assembled so that all of the strips and joints have the same relative position that they will have in the completed core, and the magnetic strips are then opened up so as to extend substantially straight out from the leg member on which the winding is being positioned.

The features of this invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and use, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is an elevation view of a bent strip core incorporating the joint arrangement of my invention;

Fig. 2 is a view of the core of Fig. 1 illustrating how my joint arrangement and electrical winding assembly method facilitates positioning of preformed electrical windings; while Fig. 3 is a view of a five-leg magnetic core of the bent strip type incorporating the joint arrangement of my invention.

Referring now to Fig. 1, there is shown a magnetic core' generally designated as 1 having three winding legs indicated as 2, 3 and 4, respectively. Each of the winding legs has positioned thereon a preformed electrical winding, with windings 5, 6 and 7 being respectively positioned on core legs 2, 3 and 4. It will be noted that a plurality of superposed strips of magnetic material which are respectively numbered 8-16, inclusive, form an inner core portion which lies partly in leg 2 and partly in leg 3, each of these turns being of generally hollow ree the turns which have been" 26-34, inclusive, form an outer core portion which surrounds the two inner core portions previously mentioned, with turns 26-34, inclusive, lying in the outer portions of the respective outer core legs 2 and 4.

In order to facilitate positioning of the preformed electrical windings 5, 6 and 7, and also to provide a composite joint which is mechanically strong, I arrange the joints of layers 8-34, inclusive, in a manner which will now be described. In accordance with my invention, successive groups of layers have their joints disposed in opposite core legs. Thus, for example, the joints for layers 8, 9 and 10, respectively, lie in the upper portion of center core leg 3, the joints for these respective layers being preferably offset in a stepped arrangement so that the joints will not all lie on the same cross-sectional line. The joints for layers'l'l, 12 and 13 lie in the upper portion of left-hand core leg 2; the joints for layers 14, and 16 lie in the upper portion of the center eore leg.

In a similar manner, the joints for layers 17, 18 and 19 lie in the upper portion of right-hand core leg 4; the joints for layers 20, 21 and 22 lie in the upper portion of center core leg 3; while the joints for layers 23, 24 and 25 he in the upper portion of right-hand core leg 4.

In the outer core section defined by the layers 2644, inclusive, the joints for layers 26, 27 and 28 lie in the upper portion of left-hand core leg 2; the joints for layers 29,30 and 31 lie in right-hand core leg 4; while the joints for layers 32, 33 and 34 lie in the left-hand core My method of assembling preformed electrical windings on a magnetic core having the joint arrangement hereinbefore described can best be seen in Fig. 2 which shows a view of the core of Fig. l with the layers pulled apart to permit positioning of preformed electrical windings.

It will be noted in Fig. 2 that when the preformed electrical windings are being positioned all of the magnetic strips of the core are in place and the core is opened up at the joints and the strips are extended so as to be in substantial alignment with the respective core legs in which they are not jointed. This permits positioning of preformed electrical windings on the respective core legs by sliding the windings down over the extended portions of the strips in alignment with a given core leg. Thus, referring to Fig. 2, preformed electrical winding 5 is placed in position, by sliding it down over the extended or opened strips of layers 8, 9 10, 14, 15, 16, 29, and 31. Similarly, in positioning coil 6 in place, this coil is slid down over the extended or opened turns of layers 11, 12, 13, 17, 18, 19, 23, 24 and 25. To position coil 7 in place, this coil is slid down over the extended or opened turns of layers 20, 21, 22, 26, 27, 28, 32, 33 and 34. Mother words, in accordance with my invention, only a fraction of the total turns lying in any given core leg have their opened-up or extended portions in alignment with that leg, thus greatly facilitating the positioning of a preformed electrical winding on any given core leg. It should be readily apparent that it is much simpler to position preformed electrical winding 5, for example, on core leg 2 when the joints are distributed as hereinbefore described than would be the case if all of the layers lying in core leg 2, for example, had their respective extended portions aligned with that core leg.

There is shown in Fig. 3 an embodiment of my invention as applied to a five-leg magnetic core of the bent strip type. This core comprises four generally hollow rectangular core members each formed of a plurality of superposed bent strip members of magnetic material. These core units are indicated by the numbers 35, 36, 37 and 38, respectively. Core unit 35 comprises leg members 39 and 40, core unit 36 comprises leg members 41 and 42, core unit 37 comprises leg members 43 and 44, and core unit 38 comprises leg members 45 and 46. Leg members 40 and 41 of core units 35 and 36, respectively. are placed in abutting relation in such manner as to provide a leg upon which the preformed winding 47 is positioned. Leg members 42 and 43 of core units 36 and 37, respectively, are placed in abutting relation in such manner as to provide a leg upon which preformed electrical winding 48 is positioned. Leg members 44 and 45 of core units 37 and 38, respectively, are positioned in abutting relation to each other in such manner as to provide a leg upon which preformed electrical winding 49 may be positioned.

All of the joints for core unit 35 are positioned in the upper portion of right-hand core leg 40 to facilitate positioning preformed electrical winding 47. This can be done since there is no electrical winding positioned on leg 39. Thus, when coil 47 is being positioned, core unit 35 may be opened up at the joints which all lie in leg 40 so that there are no extended strip portions in alignment with leg 40 over which winding 47 must be slid into position.

In. core unit 36, the joints are divided between legs 41 and 42, so that a portion of the joints lie in each of these logs. Thus, in the embodiment illustrated in the drawing, the joints for the three radially outermost layers of core unit 36 are positioned at the upper end of leg 41, and the joints for the next three or intermediate layers are positioned at the upper portion of leg 42. In core unit'37, the joints for the three radially outermost layers are positioned in leg 43, while the joints for the next three or intermediate layers are positioned in leg 44. It will be noted that the three radially innermost layers of both core units 36 and 37 are jointed in each of the core legs lying in the respective core units. Thus, the distributed joint arrangement in accordance with my invention which is applicable to magnetic layers having only one joint per layer may be used in a magnetic core structure having other types of joint arrangements.

All of the joints for core unit 38 are positioned in leg 45 to facilitate positioning of preformed electrical winding 49. Due to the fact that all of the joints for core unit 38 are in leg 45, there are no extended strip portions in alignment with leg 45 when core unit 38 is opened up for positioning of electrical winding 49.

While I have shown all of the joints of the embodiments of Figs. 1, 2 and 3 positioned at the upper end of the respective core legs, it is not necessary in practicing my invention that the joints lie in any particular region of a given core leg, or that all of the joints lying in a given core leg be positioned in the same general region of that core leg since the joints lying in a given core leg may be distributed along that core leg.

Furthermore, while I have shown the joints of alternate groups of magnetic strips disposed in opposite core legs, I may instead dispose the joints of alternate strips in 0pposite core legs.

It can be seen that in accordance with my invention whenever a plurality of superposed layers of strip magnetic material thread a pair of preformed electrical windings which are disposed on opposite core legs, the joints for the plurality of magnetic strips are distributed in such manner that a portion of the joints lie in each of the respective core legs.

It can be seen that the distributed joint arrangement described hereinbefore greatly facilitates the placement of preformed electrical windings on magnetic cores having a plurality of layers of strip magnetic material the type having only one joint per layer since it reduces the cross sectional area of the extended strip portions in alignment with any given core leg.

Furthermore, a joint arrangement in accordance with my invention in which joints are distributed alternately in oppositely-disposed core legs has greater mechanical strength than a joint arrangement in which each layer is jointed in the same core leg. This is due to the fact that the overlaps between layers having joints lying in oppositely-disposed core legs are much greater than between layers jointed in the same core leg, with a resulting greater frictional force tending to hold the composite joint together.

When my core is constructed of a plurality of precut fiat strips of magnetic material which are bent to form a closed magnetic circuit, with each layer consisting of a single strip having its ends butt jointed to each other, the further advantage results that after the core is once bent into shape, each strip remains in the same position. The flat strip members are so arranged with respect to each other that when they are bent upon themselves to form a closed loop, the joints are so distributed that a substantial portion of the joints lies in each core leg. Thus, in order to provide the distribution of joints between the core legs it is not necessary to remove strips and reassemble them, as would be necessary, for example, in the case of a wound core which is cut in a single place transversely of the core. It is advantageous not to have to reassemble the strips, since as a practical matter difficulty may be experienced in reassembling strips which were originally part of a Wound core due to the fact that different strip turns may have varying surface characteristics such as warped portions, for example, which may render difficult the positioning of the strips in some position other than that which they originally occupied in the wound core.

Furthermore, as has been pointed out, the method of assembling preformed electrical windings in accordance with my invention has the advantage that when the preformed electrical windings are being assembled on the core all of the strip members of the core are in their normal superposed relation, with each layer being opened at one place. This permits each winding to be slipped down on to a core leg which is a complete unit, and after the windings have been positioned all that remains to-be done is to close the joints.

While there havevbeen shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of manufacturing a magnetic core of the flatwise bent strip type and assembling preformed electrical windings on each leg thereof which comprises cutting a plurality of flat strips of magnetic material in progressively increasing length, stacking said strips in groups of a plurality of said strips, bending each group of strips to form a closed magnetic loop in which the opposite ends of each strip are butt jointed to each other, radially nesting said groups of bent strips with respect to each other to form a closed magnetic circuit having a pair of oppositely disposed core legs with the joints of adjacent groups lying in opposite core legs, opening said core at all of its joints and extending said groups of strip members so that they are substantially in alignment with the respective core leg from which they extend, slipping preformed electrical windings over said extended groups and onto each of the respective core legs, and reclosing said core to reform said closed magnetic loops.

2. The method of claim 1 wherein there are three strips in each group of strips.

3. The method of claim 1 wherein the joints of each assembled group of strips are positioned in staggered spaced relation with respect to each other.

References Cited in the file of this patent UNITED STATES PATENTS 1,935,426 Acly Nov. 14, 1933 2,344,294 Evans Mar. 14, 1944 2,483,159 Somerville Sept. 27, 1949 2,534,312 Somerville Dec. 19, 1950 2,548,628 Somerville Apr. 10, 1951 2,613,430 Sefton et al Oct. 14, 1952 FOREIGN PATENTS 106,986 Great Britain June 14, 1917 

1. THE METHOD OF MANUFACTURING A MAGNETIC CORE OF THE FLATWISE BENT STRIP TYPE AND ASSEMBLING PREFORMED ELECTRICAL WINDINGS ON EACH LEG THEREOF WHICH COMPRISES CUTTING A PLURALITY OF FLAT STRIPS OF MAGNETIC MATERIAL IN PROGRESSIVELY INCREASING LENGTH, STACKING SAID STRIPS IN GROUPS OF A PLURALITY OF SAID STRIPS, BENDING EACH GROUP OF STRIPS TO FORM A CLOSED MAGNETIC LOOP IN WHICH THE OPPOSITE ENDS OF EACH STRIP ARE BUTT JOINTED TO EACH OTHER, RADIALLY NESTING SAID GROUPS OF BENT STRIPS WITH RESPECT TO EACH OTHER TO FORM A CLOSED MAGNETIC CIRCUIT HAVING A PAIR OF OPPOSITELY DISPOSED CORE LEGS WITH THE JOINTS OF ADJACENT GROUPS LYING IN OPPOSITE CORE LEGS, OPENING SAID CORE AT ALL OF ITS JOINTS AND EXTENDING SAID GROUPS OF STRIP MEMBERS SO THAT THEY ARE SUBSTANTIALLY IN ALIGNMENT WITH THE RESPECTIVE CORE LEG FROM WHICH THEY EXTENDING, SLIPPING PREFORMED ELECTRICAL WINDINGS OVER SAID EXTENDED GROUPS AND ONTO EACH OF THE RESPECTIVE CORE LEGS, AND RECLOSING SAID CORE TO REFORM SAID CLOSED MAGNETIC LOOPS. 